Carbon is one of the most important known elements and can be combined with oxygen, hydrogen, nitrogen and the like. Carbon has four known unique crystalline structures including diamond, graphite, fullerene and carbon nanotubes. In particular, carbon nanotubes refer to a helical tubular structure grown with a single wall or multi-wall, and commonly referred to as single-walled nanotubes (SWNTs), or multi-walled nanotubes (MWNTs), respectively. These types of structures are obtained by rolling a sheet formed of a plurality of hexagons. The sheet is formed by combining each carbon atom thereof with three neighboring carbon atoms to form a helical tube. Carbon nanotubes typically have a diameter in the order of a fraction of a nanometer to a few hundred nanometers.
Carbon nanotubes can function as either a conductor, like metal, or a semiconductor, according to the rolled shape and the diameter of the helical tubes. With metallic-like nanotubes, it has been found that a one-dimensional carbon-based structure can conduct a current at room temperature with essentially no resistance. Further, electrons can be considered as moving freely through the structure, so that metallic-like nanotubes can be used as ideal interconnects. When semiconductor nanotubes are connected to two metal electrodes, the structure can function as a field effect transistor wherein the nanotubes can be switched from a conducting to an insulating state by applying a voltage to a gate electrode. Therefore, carbon nanotubes are potential building blocks for nanoelectronic devices because of their unique structural, physical, and chemical properties.
Existing methods for the production of nanotubes, include arc-discharge and laser ablation techniques. Unfortunately, these methods typically yield bulk materials with tangled nanotubes. Recently, reported by J. Kong, A. M. Cassell, and H Dai, in Chem. Phys. Lett. 292, 567 (1988) and J. Hafner, M. Bronikowski, B. Azamian, P. Nikoleav, D. Colbert, K. Smith, and R. Smalley, in Chem. Phys Lett. 296, 195 (1998) was the formation of high quality individual single-walled carbon nanotubes (SWNTs) demonstrated via thermal chemical vapor deposition (CVD) approach, using Fe/Mo or Fe nanoparticles as a catalyst. The CVD process has allowed selective growth of individual SWNTs, and simplified the process for making SWNT based devices. However, the choice of catalyst materials that can be used to promote SWNT growth in a CVD process has been limited to only Fe/Mo nanoparticles. Furthermore, the catalytic nanoparticles were usually derived by wet chemical routes, which are time consuming and difficult to use for patterning small features.
Another approach for fabricating nanotubes is to deposit metal films using ion beam sputtering to form catalytic nanoparticles. In an article by L. Delzeit, B. Chen, A. Cassell, R. Stevens, C. Nguyen and M. Meyyappan in Chem. Phys. Lett. 348, 368 (2002), CVD growth of SWNTs at temperatures of 900° C. and above was described using Fe or an Fe/Mo bi-layer thin film supported with a thin aluminum under layer. However, the required high growth temperature prevents integration of CNTs growth with other device fabrication processes.
Ni has been used as one of the catalytic materials for formation of SWNTs during laser ablation and arc discharge process as described by A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. H. Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tomanet, J. E. Fischer, and R. E. Smalley in Science, 273, 483 (1996) and by D. S. Bethune, C. H. Kiang, M. S. de Vries, G. Gorman, R. Savory, J. Vazquez, and R. Beyers in Nature, 363, 605 (1993). However, attempts to use a solution based Ni catalyst to grow individual SWNTs by CVD has not been successful. Additionally, although a thin Ni film has been widely used to produce MWNTs, growth of SWNTs using a Ni thin film as the active catalyst film layer has not been achieved.
A carbon nanotube is known to be useful for providing electron emission in a vacuum device, such as a field emission display. The use of a carbon nanotube as an electron emitter has reduced the cost of vacuum devices, including the cost of a field emission display. The reduction in cost of the field emission display has been obtained with the carbon nanotube replacing other electron emitters (e.g., a Spindt tip), which generally have higher fabrication costs as compared to a carbon nanotube based electron emitter.
The manufacturing costs for vacuum devices (e.g., a field emission display) that use a carbon nanotube can be further reduced if the carbon nanotube is grown on the field emission substrate from a catalytic surface using chemical vapor deposition or other film deposition techniques. Nanotube growth can be conducted as a last deposition process preventing the degradation of the electron emitter properties by other device processing techniques or steps (e.g., wet processes).
Accordingly, it is desirable to provide a porous metal catalyst on a substrate for carbon nanotube growth. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.